To James David Forbes 11 October 
Down near Bromley Kent
I venture to take the liberty of addressing you, knowing how much you are interested on the subject of your discovery on the zoned structure of glacier-ice.1 I have a specimen (from Mr Stokes’ Collection) of Mexican obsidian,2 which judging from your description, must resemble to a considerable degree, the zoned ice. It is zoned with quite straight, parallel lines, like an agate, & these zones as far as I can see under the microscope, appear entirely due to the greater or lesser number of excessively minute flattened air-cavities. I cannot avoid suspecting that in this case, & in many others, in which lavas of the trachytic series (generally of very imperfect fluidity) are laminated in a very singular manner, that this structure is due to the stretching of the mass or stream during its movement, as in the ice-streams of glaciers.3
It has occurred to me, that you possibly might like to see the specimen of obsidian & some curious, most finely, laminated obsidians & trachytic rocks, which I collected at Ascension island. You would not of course, I presume, think it worth the expence of carriage to have the specimens sent to Edinburgh; but shd you at any time come to London, I should be proud, if you so like, to send them for your inspection. If the subject of the lamination of volcanic rocks should interest you, I would venture to ask you to refer to p. 65–72 of my small volume, “Geolog: Obser: on Volcanic Isld ” which would be in the Public Library.4 I there throw out the idea, that the structure in question may, perhaps, be explained by your views on the zoned structure of glacier-ice:5 the layers of less tension, I may add, being in the case of the Ascension Obsidian-rocks, rendered apparent chiefly by the crystalline & concretionary action superinduced in them, instead of as in your ice, by the congelation of water.6
I hope you will excuse, should you feel no interest in this subject, the liberty I take in writing to you, & I beg to remain, your obedient servant | Charles Darwin
Discusses a specimen of Mexican obsidian with an unusual laminated structure.
Please cite as
Darwin Correspondence Project, “Letter no. 781,” accessed on 11 December 2016, http://www.darwinproject.ac.uk/DCP-LETT-781